FMRI reveals neural activity overlap between adult and infant pain

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FMRI reveals neural activity overlap between adult and infant pain
SHORT REPORT
          elifesciences.org

                                   fMRI reveals neural activity overlap
                                   between adult and infant pain
                                   Sezgi Goksan1, Caroline Hartley2, Faith Emery2, Naomi Cockrill2, Ravi Poorun1,
                                   Fiona Moultrie2, Richard Rogers1, Jon Campbell1, Michael Sanders1, Eleri Adams2,
                                   Stuart Clare1, Mark Jenkinson1, Irene Tracey1, Rebeccah Slater1,2*
                                   1
                                    Oxford Centre for Functional Magnetic Resonance Imaging of the Brain, Nuffield
                                   Department of Clinical Neurosciences, University of Oxford, Oxford, United
                                   Kingdom; 2Department of Paediatrics, University of Oxford, Oxford, United Kingdom

                                   Abstract Limited understanding of infant pain has led to its lack of recognition in clinical practice.
                                   While the network of brain regions that encode the affective and sensory aspects of adult pain are
                                   well described, the brain structures involved in infant nociceptive processing are less well known,
                                   meaning little can be inferred about the nature of the infant pain experience. Using fMRI we
                                   identified the network of brain regions that are active following acute noxious stimulation in newborn
                                   infants, and compared the activity to that observed in adults. Significant infant brain activity was
                                   observed in 18 of the 20 active adult brain regions but not in the infant amygdala or orbitofrontal
                                   cortex. Brain regions that encode sensory and affective components of pain are active in infants,
                                   suggesting that the infant pain experience closely resembles that seen in adults. This highlights the
                                   importance of developing effective pain management strategies in this vulnerable population.
                                   DOI: 10.7554/eLife.06356.001

                                   Introduction
                                   The network of brain regions that encode both the affective and sensory aspects of the pain
                                   experience have been well described in the adult (Apkarian et al., 2005; Tracey and Mantyh, 2007).
*For correspondence: rebeccah.
slater@paediatrics.ox.ac.uk        It is not known which cortical and subcortical brain structures are activated following noxious events in
                                   infants. Early evidence demonstrated that infants exhibited reflex responses and concluded that pain
Competing interests: The           was not processed at the level of the cortex (Rodkey and Pillai Riddell, 2013). This, coupled with an
authors declare that no
                                   infant’s inability to describe their pain experience verbally, led to extreme controversy regarding
competing interests exist.
                                   whether an infant has the ability to experience the unpleasant affective components of pain (Rodkey
Funding: See page 11               and Pillai Riddell, 2013). Consequently, infants have received poor pain management, exemplified
Received: 06 January 2015
                                   during the 1980s by surgery being routinely performed using neuromuscular blocks without provision
Accepted: 11 March 2015            of adequate analgesia (Anand and Hickey, 1987). More recent research has primarily focussed on
Published: 21 April 2015           behavioural and physiological measures, which has led to the development of a number of infant pain
                                   assessment tools (Duhn and Medves, 2004). However, the lack of sensitivity and specificity of these
Reviewing editor: Jody C
                                   measures means the trend to undertreat pain remains in clinical practice (Carbajal et al., 2008),
Culham, University of Western
                                   despite concerted efforts to improve the management of pain in this population (Anand and
Ontario, Canada
                                   International Evidence-Based Group for Neonatal Pain, 2001). For example, it is remarkable that
    Copyright Goksan et al. This   current UK NHS guidelines for ankyloglossia (tongue tie) surgery state that ‘in small babies, being
article is distributed under the   cuddled and fed are more important than painkillers’ (NHS Choices, 2015). Indeed, a recent review of
terms of the Creative Commons
                                   neonatal pain management practice in intensive care highlighted that although infants experience an
Attribution License, which
                                   average of 11 painful procedures per day, 60% of the population did not receive any pharmacological
permits unrestricted use and
                                   analgesia (Roofthooft et al., 2014).
redistribution provided that the
original author and source are          Recent studies using EEG and near-infrared spectroscopy have been used to provide
credited.                          reliable evidence that nociceptive information is transmitted to the newborn infant brain

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                          1 of 13
FMRI reveals neural activity overlap between adult and infant pain
Short report                                                                                                      Neuroscience

                                   eLife digest Doctors long believed that infants do not feel pain the way that older children and
                                   adults do. Instead, they believed that the infants’ responses to discomfort were reflexes. Based on
                                   these beliefs, it was a routine practice to perform surgery on infants without suitable pain relief up
                                   until the late 1980s. Even now, infants may receive less than ideal pain relief. For example, a review
                                   found that although newborns in intensive care units undergo 11 painful procedures per day on
                                   average, more than half of the babies received no pain medications. Some guidelines continue to
                                   emphasize that for infants cuddling and feeding are more important sources of comfort than pain-
                                   relieving drugs.
                                       There is growing support for better pain control for infants. Doctors and nurses now routinely
                                   observe behaviour and physiological responses—such as heart rate—to assess whether infants are
                                   experiencing pain. When an infant shows signs of pain, medical staff may give the infant sugar water
                                   or other interventions aimed at reducing their distress. However, recordings of brain activity suggest
                                   that infants may experience pain without exhibiting physical signs and that sugar water may reduce
                                   the behaviours associated with pain but not the pain itself.
                                       More objective measurements of infant pain would be useful, but to create such measure-
                                   ments scientists must first understand how infants experience pain. So Goksan et al. used
                                   a technique called functional magnetic resonance imaging (fMRI) to compare the brain responses
                                   of adults and newborns to the same stimulus—a sharp poke of the foot. The adults were also
                                   asked about the pain they experienced, and whether the infants pulled their foot away when
                                   poked was documented.
                                       The fMRI results revealed that pain increased activity in 20 regions in the adults’ brains, and 18
                                   of the same regions in the infants’ brains. The brain regions activated in the infants’ brains in
                                   response to a poke on the foot are involved in processing sensations and emotions. The two
                                   regions that did not activate in the infant brains—the amygdala and the orbitofrontal
                                   cortex—help individuals interpret the stimuli. Goksan et al. therefore conclude that infants
                                   experience pain in similar ways to adults, though they may not experience all the emotions that
                                   adults have when they are in pain. It is, therefore, important to give infants suitable pain relief
                                   during potentially painful procedures.
                                   DOI: 10.7554/eLife.06356.002

                                   (Slater et al., 2006, 2010a, 2010b), and have highlighted the limitations of using observational
                                   behavioural measures to quantify pain in infants. For example, nociceptive information can be
                                   processed in the infant brain without a concomitant behavioural response (Slater et al., 2008),
                                   and interventions thought to alleviate pain (i.e., oral sucrose) can reduce clinical pain scores without
                                   reducing evoked nociceptive brain activity (Slater et al., 2010a). While these studies confirm
                                   that the infant central nervous system can process noxious stimulation, they do not elucidate the
                                   nature of the infant experience—in particular, which brain regions are involved, and therefore,
                                   whether the sensory, cognitive, and emotional aspects of pain are present in this population.
                                   Here, we identify the cortical and subcortical structures activated following acute noxious
                                   stimulation in the healthy newborn infant, and compare the activity with that observed in adults.
                                   The feasibility of this approach was demonstrated in a foundational pilot study (Williams et al.,
                                   2015). A case study in a single infant demonstrated that noxious stimulation evoked widespread
                                   brain activity (Williams et al. 2015), which included brain regions previously reported to be
                                   involved in adult pain (Tracey et al., 2007). Using a reverse inference approach to compare
                                   active brain regions in infants with those reported during adult pain, we postulate which aspects
                                   of the pain experience are present (Wager et al., 2013), providing an opportunity to gain insight
                                   into the organisation of nociceptive circuitry in the naı̈ve infant brain.
                                      In this study, acute noxious stimulation (PinPrick Stimulators, MRC Systems) was applied to the foot
                                   in both adults (n = 10; applied force: 32–512 mN) and infants (n = 10; applied force: 32–128 mN;
                                   greater force was not applied due to the potential risk of tissue damage). Using functional magnetic
                                   resonance imaging (fMRI) changes in blood oxygen level dependent (BOLD) activity in the brain were
                                   recorded in response to the stimuli. Adults were asked to verbally report their pain intensity and,
                                   using the McGill Pain questionnaire (Melzack and Torgerson, 1971), to describe the quality of the

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                         2 of 13
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                                   pain they experienced. As infants are unable to describe their pain, reflex leg withdrawal from the
                                   stimuli was visually observed during scanning. Parents were present during the studies and no infants
                                   were withdrawn from the study after recruitment.

                                   Results and discussion
                                   Adult participants reported increased pain with increasing stimulus intensity (r = 0.48; p < 0.0001),
                                   and most frequently described the pain as pricking (n = 8 of 10) and sharp (n = 6 of 10). In infants,
                                   application of the stimuli evoked visible withdrawal of the stimulated leg, which could be observed at
                                   all stimulus intensities, whereas in adults, reflex withdrawal of the leg or foot was not observed at any
                                   stimulus intensity. While low threshold stimuli can also evoke reflex withdrawal in infants (Cornelissen
                                   et al., 2013), this observation confirms that the stimuli applied in this study were detected by the
                                   peripheral nervous system and transmitted to the central nervous system. Although noxious
                                   stimulation can elicit reflex limb withdrawal in adults, supraspinal modulation of the input means this
                                   activity is often suppressed in experimental studies.
                                       In adults, noxious stimulation evoked significant increases in BOLD activity in cortical and
                                   subcortical brain regions, including primary somatosensory cortices, anterior cingulate cortex (ACC),
                                   bilateral thalamus, and all divisions of the insular cortices (Figure 1). All brain regions that had
                                   a significant increase in BOLD following noxious stimulation are identified in Table 1, and are
                                   consistent with previous literature (Tracey and Mantyh, 2007). In infants, the increases in BOLD
                                   activity evoked by the noxious stimulation were extremely similar to that seen in adults, and all but two
                                   of the 20 regions that were active in the adults were active in infants (Table 1 and Figure 1). While in
                                   adults the parietal lobe, pallidum, and precuneus cortex were only active in the brain regions
                                   contralateral to the site of stimulation, in infants these brain regions were also active on the ipsilateral
                                   side to the stimulus. Additional brain regions that were only active in the infants included the bilateral
                                   auditory cortices, hippocampus, and caudate (Table 1). The increased bilateral activity and greater
                                   number of active regions in infants are likely due to the immature cortico-cortical and interhemispheric

                                   Figure 1. Comparison of nociceptive-evoked brain activity in selected brain regions that are active in both adults
                                   and infants. Significantly, active voxels across each stimulus intensity level are presented for (A) adult and (B) infant
                                   participants (applied force: adults 32–512 mN; infants 32–128 mN). Each colour represents activity in a different
                                   anatomical brain region. (A) Adult activity is overlaid onto a standard T1 weighted MNI template and (B) infant
                                   activity is overlaid onto a standard T2 weighted neonatal template, corresponding to a 40-week gestation infant.
                                   ACC: anterior cingulate cortex; S1: primary somatosensory cortex: PMC: primary motor cortex; SMA: supplementary
                                   motor area.
                                   DOI: 10.7554/eLife.06356.003

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                                         3 of 13
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                                   pathways (Kostovic and Jovanov-Milosevic, 2006). Major reorganisation of the cortical circuitry
                                   occurs after the first postnatal month when there is a striking retraction of exuberant axons in the
                                   corpus callosum and there is a cessation of growth of the long cortico-cortical afferent pathways
                                   (Jovanov-Milosevic et al., 2006; Kostovic and Jovanov-Milosevic, 2006).
                                       Although the infant brain activity was widespread, the specificity of the response was
                                   demonstrated, as it was not present across all brain regions. For example, brain regions not
                                   commonly associated with the cerebral processing of nociceptive stimulation in the adult, such as the
                                   olfactory cortex, cuneus, and fusiform gyrus, were also not active in the infants. 14% of voxels across
                                   the whole brain were active following the application of the 128 mN stimuli in infants compared with
                                   9% of voxels following the 512 mN stimuli in adults (Figure 2). In contrast, the 128 mN stimulus
                                   activated less than 1% of voxels in the adult brain. This demonstrates that the coverage and
                                   distribution of brain activity evoked by the 128 mN stimulus in infants was most similar to that
                                   evoked by the 512 mN stimulus in adults (Figure 2). This suggests that infants have increased
                                   sensitivity to nociceptive stimuli compared with adults, which is supported by previous data that
                                   show spinal nociceptive reflex withdrawal activity has greater amplitude and duration in infants
                                   compared with adults (Andrews and Fitzgerald, 1999; Skljarevski and Ramadan, 2002;
                                   Cornelissen et al., 2013). These data strongly imply that the threshold for evoking widespread
                                   nociceptive brain activity in infants is substantially lower than in adults. It is, however, not known
                                   whether the increased brain activity observed at a lower threshold in the infants is due to increased
                                   peripheral drive, for example due to differences in skin thickness between the adult and infant
                                   populations, or due to differences in transduction or subsequent central processing of the
                                   nociceptive input.
                                       Noxious stimulation in infants did not evoke activity in the amygdala or orbitofrontal cortex (OFC)
                                   (Table 1), and in contrast to the adults, where activity was present across all divisions of bilateral
                                   insular cortices, activity in the anterior division was not present (Figure 1). A recent white matter
                                   tractography study of the adult brain shows that the anterior insula has dominant connections with the
                                   OFC (Wiech et al., 2014). Based on many imaging studies spanning a range of stimuli and tasks, it is
                                   thought that activation in the anterior insula reflects the net evaluation of the affective impact of an
                                   impending situation. Similarly, the OFC is sensitive to stimuli with an emotional valence, however, it
                                   primarily responds to the reward value of the stimulus (including negative value) rather than its
                                   sensory features. Importantly, the OFC also encodes the anticipation of future outcomes, which makes
                                   it well suited for guiding subsequent decisions (Kahnt et al., 2010). It is likely that the infants are too
                                   immature and inexperienced to evaluate and contextualise the nociceptive stimulus into a coordinated
                                   decision and response, which might account for the lack of activity within these regions. Similarly, in
                                   adults the amygdala is thought to attach emotional significance to the nociceptive inputs it receives,
                                   and to play a role in fear and anxiety (Simons et al., 2014), which may reflect affective qualities that
                                   the newborn infant does not yet ascribe to the stimulus.
                                       In light of these observations, it is plausible that infants do not experience the full range of aversive
                                   qualities that adults associate with nociceptive input. Indeed, this hypothesis is supported by evidence
                                   from rat pups, which shows that avoidance behaviour in a fear-conditioning paradigm does not manifest
                                   until postnatal day 10, and is associated with the enhancement of neural activity within the amygdala
                                   (Sullivan et al., 2000; Sullivan, 2001). Nevertheless, the observation that brain structures involved in
                                   affective processing, such as the anterior cingulate cortex, are activated following noxious stimulation
                                   suggests that infants do have the capacity to experience an emotionally relevant context related to
                                   incoming sensory input. Indeed, in adults the modulation of pain-related activity in the anterior cingulate
                                   cortex closely parallels a selective change in perceived unpleasantness (Rainville et al., 1997).
                                       11 brain regions significantly encoded stimulus intensity in adults, whereas none of the active
                                   regions in infants exhibited significant intensity encoding (Table 1). Although the trend for intensity
                                   encoding in infants is clearly evident in some brain regions, these data suggest that infants do not
                                   discriminate stimulus intensity as well as adults (Figure 2—figure supplement 1). As only three
                                   stimulus intensities were applied to the infants it is plausible that if the intensity range were increased,
                                   significant intensity encoding may be observed. Nevertheless, when considering adult brain regions
                                   that did significantly intensity encode, three of the four highest ranked brain regions (ranked
                                   according to the degree of intensity encoding, and identified as the contralateral temporal gyri,
                                   opercular cortex, and all divisions of the insular cortex), were ranked in the same order within the top
                                   three regions in infants, highlighting the remarkable similarity in how the immature infant brain and

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                              4 of 13
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                                   adult brain encode nociceptive information (Table 1). Intensity encoding has been reported following
                                   low intensity von Frey hair stimulation (Williams et al., 2015).
                                      Inferences about the subjective experience of pain are highly speculative, whether based on brain
                                   imaging data, behavioural responses or other autonomic or physiological observations. In most adults,
                                   where the pain experience can be communicated verbally, it is not always necessary to rely on
                                   surrogate measures when attempting to quantify an individual’s pain experience or when assessing
                                   the need for analgesic provision. However, where verbal report is not possible as in the infant
                                   population or in those who are cognitively impaired, reliance on surrogate measures is essential when
                                   making inferences about pain perception. As cortical activation is a fundamental requirement for an
                                   experience to be interpreted as painful, inferences based on patterns of brain activity may provide the
                                   most reliable surrogate measure of pain compared with alternative approaches based on behavioural
                                   and physiological indicators that may not be reliably linked to central sensory or emotional processing
                                   in the brain (Oberlander et al., 2002; Ranger et al., 2007). This does not, however, negate the
                                   importance of taking a multidimensional approach to infant pain assessment by considering
                                   measures of brain activity in the context of other well-characterised behavioural and physiological
                                   indicators. Indeed, some researchers have argued that reverse inference based on brain imaging
                                   results should be used merely as a guide to direct further enquiry rather than a direct means to
                                   interpret results (Poldrack, 2008). Nevertheless, it has been shown using multivariate pattern
                                   analysis that pain-related brain activity can be classified and discriminated from other
                                   psychological states, suggesting a neural state for pain perception that is distinct from other
                                   sensory modalities and affective experiences (Yarkoni et al., 2011; Wager et al., 2013). Although
                                   we cannot necessarily infer an infant’s subjective experience based on a given pattern of brain
                                   activity, these results make certain conclusions more likely. The closer the pattern of brain activity
                                   mimics activity observed in adults—who can report their subjective experience—the stronger the

                                   Figure 2. Noxious-evoked brain activity in response to the maximal presented stimulus in adults (512 mN) and
                                   infants (128 mN). Red-yellow coloured areas represent active brain regions (threshold z ≥ 2.3 with a corrected cluster
                                   significance level of p < 0.05). An image of a midline sagittal brain slice (right panel) identifies the location of each
                                   example slice in the horizontal plane. (A) Adult activity is overlaid onto a standard T1 weighted MNI template and (B)
                                   infant activity is overlaid onto a standard T2 weighted neonatal template, corresponding to a 40-week gestation
                                   infant.
                                   DOI: 10.7554/eLife.06356.005
                                   The following figure supplement is available for figure 2:
                                   Figure supplement 1. Relationship between percentage change in BOLD signal and stimulus intensity (force) in four
                                   example active brain regions in adult and infant participants.
                                   DOI: 10.7554/eLife.06356.006

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                                         5 of 13
Table 1. Identification of all active brain regions in adults and infants following acute noxious stimulation at all stimulus intensities (applied force: adults 32–512 mN;
                                                              infants 32–128 mN)
                                                                                                                                                        Adults                                               Infants
                                                                                                                                                                                              Neonate
                                                                                                                                                                                              template
                                                                                                                               Peak Z                          Slope of            Peak Z                            Slope of
                                                                                                                                          MNI coords                                          coords
                                                                                                    Anatomical                 within                          regression   P      within                            regression    P
                                                                                                                                                                                                                                             Short report

                                                                                                    area                Region cluster    x    y    z     Rank (*E-03)      val*   cluster    x    y     z      Rank (*E-03)       val*
                                                                                                    Temporal gyrus      Contra     3.92   64   −34 20     1      1.01       0.0002 3.05       32   −32 12       1      2.46        0.0083
                                                                                                    Cingulate gyrus     Anterior   4.11   6    4    40    2      0.65       0.0005 2.58       −1   1     26     11     1.01        0.3971
                                                                                                    Opercular cortex    Contra     5.60   40   6    10    3      0.63       0.0001 3.38       32   −13 19       2      2.23        0.0391
                                                                                                    Insula              Contra     4.18   34   14   6     4      0.61       0.0001 3.04       19   −22 23       3      2.15        0.0207
                                                                                                    Supramarginal       Contra     4.33   64   −38 20     5      0.60       0.0008 3.29       25   −23 39       9      1.08        0.1749
                                                                                                    gyrus
                                                                                  Intensity
                                                                                                   Postcentral gyrus    Contra     4.28   58   −18 22     6      0.60       0.0012 3.85       15   −22 52       10     1.01        0.2667
                                                                                  encoding regions
                                                                                  (in adults)      Visual cortex        Contra     3.62   44   −62 4      7      0.59       0.0004 3.25       21   −52 34       6      1.41        0.0814
                                                                                                    Putamen             Contra     3.68   22   6    6     8      0.55       0.0001 3.30       17   −17 18       8      1.20        0.1656

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356
                                                                                                    Thalamus            Contra     3.51   14   −14 0      9      0.50       0.0010 3.58       6    −16 15       4      1.91        0.0592
                                                                                                    Insula              Ipsi       4.67   −38 −18 14      10     0.49       0.0001 2.59       −26 −14 14        5      1.69        0.1015
                                                                                                    Supplementary       Contra     3.91   8    4    46    11     0.39       0.0008 3.50       6    −18 48       7      1.23        0.2315
                                                                                                    motor area
                                                                                                    Cerebellum          Ipsi       3.88   −20 −66 −44            0.35       0.0029 3.53       −3   −46 −6              3.57        0.0164
                                                                                                    Temporal gyrus      Ipsi       3.72   −52 −56 10             0.18       0.5487 3.41       −32 −22 14               2.90        0.0196
                                                                                                    Supramarginal       Ipsi       4.59   −64 −28 20             0.51       0.0035 3.13       −31 −24 30               2.79        0.0055
                                                                                                    gyrus
                                                                                                    Cerebellum          Contra     3.36   20   −70 −50           0.31       0.0246 3.16       2    −44 −6              2.72        0.1634
                                                                                                    Opercular cortex    Ipsi       5.23   −50 −28 26             0.50       0.0018 2.69       −27 −12 13               2.23        0.0710
                                                                                                    Postcentral gyrus   Ipsi       4.71   −62 −18 24             0.44       0.0375 3.52       −31 −15 41               2.12        0.0845
                                                                                                    Thalamus            Ipsi       3.52   −12 −14 10             0.42       0.0018 3.48       −1   −20 13              1.67        0.1009
                                                              Active regions in
                                                                                                    Angular gyrus       Ipsi       3.59   −58 −50 18             0.53       0.0107 2.98       −23 −39 33               1.56        0.0528
                                                              both adults and     Non intensity
                                                              infants             encoding regions Precentral gyrus     Ipsi       4.01   −58 0     10           0.43       0.0578 3.46       −23 −17 48               1.53        0.1247
                                                                                  (in adults)      Frontal gyrus        Contra     3.88   58   12   0            0.56       0.0212 3.11       11   −12 48              1.42        0.0646
                                                                                                    Cingulate gyrus     Posterior 3.71    −14 −28 38             0.08       0.2480 3.18       −9   −23 35              1.42        0.1101
                                                                                                    Angular gyrus       Contra     3.71   60   −46 18            0.54       0.0080 3.12       22   −51 35              1.42        0.0407
                                                                                                    Precuneous          Contra     3.60   16   −68 40            0.38       0.0714 3.70       5    −30 52              1.19        0.1623
                                                                                                    cortex
                                                                                                    Visual cortex       Ipsi       3.82   −52 −70 10             −0.09      0.3758 2.59       −7   −40 11              1.17        0.1657
                                                                                                    Brainstem                      3.86   10   −26 −8            0.33       0.1710 2.99       −3   −27 −10             1.11        0.4350
                                                                                                    Parietal lobule     Contra     3.10   20   −44 68            0.61       0.1097 3.10       27   −24 46              1.09        0.1271

                                                              Table 1. Continued on next page

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Table 1. Continued
                                                                                                                                                               Adults                                                        Infants
                                                                                                                                                                                                             Neonate
                                                                                                                                                                                                             template
                                                                                                                                Peak Z                                Slope of                    Peak Z                             Slope of
                                                                                                                                                MNI coords                                                   coords
                                                                                                     Anatomical                 within                                regression         P        within                             regression       P
                                                                                                     area                Region cluster         x     y    z     Rank (*E-03)            val*     cluster    x     y     z      Rank (*E-03)          val*
                                                                                                                                                                                                                                                                 Short report

                                                                                                     Putamen             Ipsi       3.63        −16 10     −2             0.45           0.0023 3.13         −14 −14 19                 0.92          0.2813
                                                                                                     Supplementary       Ipsi       3.55        −6    4    44             0.40           0.0219 3.16         −4    −10 46               0.91          0.3903
                                                                                                     motor area
                                                                                                     Precentral Gyrus    Contra     4.05        58    4    8              0.44           0.0276 3.76         6     −20 53               0.88          0.2672
                                                                                                     Frontal gyrus       Ipsi       3.57        −8    22   32             −0.24          0.1954 2.79         −13 −9      50             0.70          0.4820
                                                                                                     Pallidum            Contra     3.40        16    −4   −4             0.49           0.0071 2.84         13    −13 13               0.64          0.4863
                                                                                                     Amygdala            Contra     3.49        20    −2   −14            0.69           0.0160
                                                                                                     Amygdala            Ipsi       4.28        −20 −2     −12            0.43           0.0860
                                                              Active regions in                      Orbitofrontal       Ipsi       3.40        −18 4      −16            0.42           0.0157                           no activity
                                                              adults only                            cortex
                                                                                                     Orbitofrontal       Contra     3.57        34    30                  0.44           0.0460

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356
                                                                                                                                                           −2
                                                                                                     cortex
                                                                                                     Precuneous          Ipsi                                                                     3.80       −1    −26 52               1.26          0.1699
                                                                                                     cortex
                                                                                                     Pallidum            Ipsi                                                                     3.16       −8    −5    14             0.59          0.4787
                                                                                                     Parietal lobule     Ipsi                                                                     3.31       −28 −23 33                 0.99          0.2711
                                                                                                     Auditory cortex     Contra                                                                   2.89       26    −14 18               3.07          0.0119
                                                                                                     Auditory cortex     Ipsi                                                                     3.34       −17 −29 19                 2.56          0.0304
                                                              Active regions in
                                                              infants only                           Caudate             Contra                             no activity                           3.61       13    −17 22               0.59          0.5822
                                                                                                     Caudate             Ipsi                                                                     3.47       −7    −8    18             1.05          0.3415
                                                                                                     Hippocampus         Contra                                                                   2.61       21    −25 9                1.84          0.1288
                                                                                                     Hippocampus         Ipsi                                                                     2.77       −15 −31 9                  1.00          0.3326
                                                                                                     Parahippocampus Contra                                                                       3.02       11    −23 0                1.53          0.3740
                                                                                                     Parahippocampus Ipsi                                                                         2.99       −7    −24 −8               0.19          0.9013

                                                              Active brain regions were defined as regions with more than one active voxel with significant positive BOLD activity (z = 2.3; corrected cluster significance level of p < 0.05). The intensity
                                                              encoding regions are reported with the corresponding p values and slope of the regression that refer to the degree of intensity encoding. The intensity encoding regions (in adults) are ranked
                                                              according to the slope of the regression. * Threshold for significant intensity encoding was p < 0.00156 following a Bonferroni correction.
                                                              DOI: 10.7554/eLife.06356.004

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                                   inference. The patterns of brain activity observed in this study make it likely that the infant
                                   experience is similar to that described by adults.
                                      Pain is defined as an unpleasant sensory and emotional experience. This study provides the first
                                   demonstration that many of the brain regions that encode pain in adults are also active in full-term
                                   newborn infants within the first 7 days of life. This strongly supports the hypothesis that infants are
                                   able to experience both sensory and affective aspects of pain, and emphasizes the importance of
                                   effective clinical pain management.

                                   Materials and methods
                                   Participants
                                   10 healthy adults (mean age = 28.3 years; range: 23–36) and 10 healthy term-born infants (mean
                                   gestational age at time of study = 40.6 weeks; range: 38.6–42.7) participated in the study. Adult
                                   participants were members of staff or postgraduate students at The University of Oxford, and
                                   infants were recruited from the Maternity Unit at the John Radcliffe Hospital, Oxford. At the time of
                                   study all infants were less than 7 days old (mean postnatal age = 3 days; range: 1–6). Infant
                                   participants were eligible for inclusion in the study if they were healthy, had no history of
                                   neurological problems, born after 37 weeks gestation, self-ventilating in air and clinically stable at
                                   the time of study.

                                   Recruitment
                                   Informed written consent and consent to publish the results were provided by adult participants or by
                                   the infant’s parent before the study commenced. The study was approved by the National Research
                                   Ethics Service and the University of Oxford Central University Research Ethics Committee. The study
                                   conformed to the standards set by the Declaration of Helsinki and Good Clinical Practice guidelines.
                                      A member of the research team identified infants who were eligible for inclusion in the study
                                   shortly after birth. Prior to obtaining consent for infants to take part, parents were shown the
                                   experimental stimulators and given the opportunity to test the stimulators before they were applied
                                   to the infants. A full description of the MRI scanning environment was also provided. Parents of 113
                                   infants were approached to take part in the study. 44 parents expressed an interest in the proposed
                                   research and 11 infants were recruited to the study. Parents were invited to stay with their infants
                                   during the study and in nearly all cases, one or both parents chose to do so. Parents were also
                                   informed that if their infant became restless while in the scanner, the study would be stopped.
                                      Recruitment success rate was highly dependent on infant availability during the pre-booked MRI
                                   scan slots. One study was stopped due to the baby being restless when placed on the MRI bed. In
                                   adults, 100% of the subjects (10 out of 10) who were approached to take part in the study gave their
                                   consent for the psychophysical and MRI aspects of the study.

                                   Experimental study design
                                   Functional magnetic resonance imaging (fMRI) of the brain was performed on all participants in
                                   response to acute noxious stimulation. On a second test occasion in the adults, the experimental
                                   protocol was repeated outside the scanner and the psychophysical data were recorded. During this
                                   session, participants were asked to verbally rate pain intensity using a numerical scale (0–10) and to
                                   describe the type of pain they experienced using the McGill pain questionnaire (Melzack and
                                   Torgerson, 1971).

                                   Experimental techniques
                                   Noxious stimulation
                                   Acute noxious (non-skin-breaking) stimulation was applied using graded nociceptive stimulators
                                   (PinPrick Stimulators, MRC Systems). In adults, five intensities of stimulation were applied to the
                                   dorsum of the left foot (applied force: 32, 64, 128, 256, and 512 mN). In infants, three intensities of
                                   stimulation were applied to the heel of the left foot (applied force: 32, 64, 128 mN). Greater force was
                                   not applied in infants to avoid the potential risk of tissue damage. Each stimulus was delivered 10
                                   times with a minimum inter-stimulus interval (ISI) of 25 s. In all cases, stimuli were delivered by the
                                   experimenter in one smooth motion and lasted approximately 1 s.

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                                   Recording techniques
                                   MRI study protocol
                                   Preparation
                                   All MRI scans conformed to the FMRIB (Functional Magnetic Resonance Imaging of the Brain) Centre
                                   ethical and safety guidelines. Adult participants were screened for MRI safety by a radiographer.
                                   Ear protection was provided (foam ear plugs, 3M, St. Paul, Minnesota; sound attenuation 28 dB) and
                                   adults were made comfortable while lying on the MRI scanner bed. The head was positioned inside
                                   the head coil and padding was used to restrict head movement.
                                       Infants were accompanied and transported to FMRIB by a member of clinical staff, trained in
                                   neonatal life support, who remained with each infant throughout the study to ensure the infant’s
                                   safety and wellbeing. Infants were screened for metal items (including metal poppers on clothing that
                                   were in direct contact with their skin) and were fed and swaddled before being placed on a vacuum-
                                   positioning mattress on the MRI bed. Ear putty, ear muffs (Minimuffs, Natus Medical Inc., Galway,
                                   Ireland), and ear-defenders (Em’s 4 Bubs Baby Earmuffs, Em’s 4 Kids, Brisbane, Australia) were fitted
                                   (sound attenuation levels: 23 dB, 7 dB, and 22 dB, respectively). Finally, extra padding was placed
                                   around the ear-defenders to restrict head movement. The infant’s temperature was measured before
                                   the scan commenced, and heart rate and oxygen saturation was monitored throughout the scan using
                                   a 3T MRI compatible neonatal monitoring probe placed on the right foot (Fibre Optic Pulse Oximeter;
                                   Nonin Medical, Plymouth, Minnesota). Parents who accompanied their infants were also MRI safety
                                   screened and provided with adequate ear protection, and were asked to sit inside the MRI scan room
                                   throughout the scans.

                                   MR image acquisition
                                   MRI data were acquired using a Siemens 3-Tesla Magnetom Verio system (Erlangen, Germany) with
                                   a 32-channel head coil. Anatomical scans were first acquired and if excessive motion was identified,
                                   a second acquisition was attempted. For adults, a T1-weighted sequence (MPRAGE; TR = 2040 ms; TE
                                   = 4.7 ms; flip angle 8˚; resolution 1 × 1 × 1 mm; axial slices = 192) was acquired and for infants a T2-
                                   weighted sequence (TSE; TR = 13871 ms; TE = 89 ms; flip angle 150˚; resolution 1 × 1 × 1 mm; slices =
                                   80) was used. BOLD images were acquired using a T2* weighted echo-planar imaging (EPI) sequence
                                   with an echo time (TE) optimised for either adults (TR = 3280 ms; TE = 30 ms; flip angle = 90˚; FOV =
                                   192 mm; imaging matrix 64 × 64; resolution 3 × 3 × 3 mm; slices = 50; average total volumes = 96) or
                                   infants (TR = 2500 ms; TE = 40 ms; flip angle = 90˚; FOV = 192 mm; imaging matrix 64 × 64; resolution
                                   3 × 3 × 3 mm; slices = 33; average total volumes = 136). Field map images were obtained for post-
                                   acquisition correction of gradient field effects. Prospective Acquisition Correction for head motion
                                   (PACE) was applied during all EPI scans. PACE is a motion correction technique that tracks the
                                   position of the head during scan acquisition and applies a real-time correction for large head
                                   movements (Thesen et al., 2000). The noxious stimuli were time-locked to the fMRI recording using
                                   Neurobehavioural Systems (Presentation) software that recorded each time the experimenter pressed
                                   a button while simultaneously applying the experimental stimuli to the participant’s foot.
                                      The MR data acquisition protocol was 28 min in infants and 40 min in adults. On average infants
                                   spent 60 min in the scanner room, which allowed time to prepare and settle the infants before and
                                   during scanning.

                                   Sleep state
                                   Infant sleep state could not be controlled during the study as infants fluctuated between being quietly
                                   awake and asleep. Adults were not instructed to stay awake during scanning and three adults
                                   reported that they fell asleep.

                                   Adult psychophysics and pain questionnaire
                                   Participants were asked to lie down on a patient bed. Throughout the experiment, adults were asked
                                   to verbally state a pain score following each individual stimulus using a pain scoring system where 0 is
                                   no pain and 10 is the worst pain imaginable. Once all stimuli had been presented, the participants
                                   were asked to describe the type of pain they experienced by completing the McGill Pain
                                   Questionnaire (Melzack and Torgerson, 1971).

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                         9 of 13
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                                   Data analysis
                                   MR data
                                   All MR data processing was done using the FMRIB Software Library (FSL) (www.fmrib.ox.ac.uk/fsl). FSL
                                   Version 4.9.1 (with no boundary based registration [BBR]) was used in infants and FSL Version 5.0 was
                                   used in adults (Woolrich et al., 2009). Standard preprocessing steps were performed in all fMRI data
                                   sets using the FMRI Expert Analysis Tool (FEAT, version 6.0). The FSL Brain Extraction Tool (BET) was
                                   used to remove non-brain structures from the adult and infant structural images and from the adult
                                   field map images (Smith, 2002). In the infant field maps, brain extraction was achieved using a mask
                                   of the infant’s brain-extracted structural image to guide the field map preparation. For each adult, the
                                   functional data were registered using a two-step registration: (i) the EPI image was registered to the
                                   subject’s T1-weighted structural image, with a rigid body transformation, six DOF and BBR, using
                                   FMRIBs Linear Image Registration Tool (FLIRT) (Jenkinson and Smith, 2001; Jenkinson et al., 2002;
                                   Greve and Fischl, 2009); and (ii) the T1-weighted structural image was registered to a standard MNI
                                   image (http://www.bic.mni.mcgill.ca/ServicesAtlases/ICBM152NLin6) using FMRIBs Non-linear Reg-
                                   istration Tool (FNlRT) with a non-linear transformation and 12 DOF. In each infant, the functional data
                                   were registered using a three step registration: (i) the EPI image was registered to the subjects T2-
                                   weighted structural images using FLIRT, with a rigid body transformation with six DOF and no BBR
                                   (Jenkinson and Smith, 2001; Jenkinson et al., 2002); (ii) the T2-weighted structural images were
                                   registered to a neonatal specific template image, which corresponded to the gestational age of the
                                   infant at the time of the study (Serag et al., 2012); and (iii) the template images were then registered
                                   to a standardized infant template, corresponding to a 40-week gestation infant (Serag et al., 2012).
                                   The final two stages of the infant registration were carried out using a non-linear transformation
                                   (FNIRT) and 12 DOF. The 40-week gestation template was chosen because it most closely matched
                                   the median age of the infants.
                                       Functional data were spatially smoothed (full width half maximum = 5 mm) and temporal filtering
                                   (high pass cut off = 90 s) was also applied. Motion artifacts were minimised using Motion Correction with
                                   MCFLIRT (Jenkinson et al., 2002) and by the addition of motion-derived explanatory variables (EV) in
                                   the models. A single EV was included for each volume that was identified as having a large deviation in
                                   head position (FSL motion outliers were calculated per data set using a framewise displacement),
                                   effectively removing the signals associated with the identified timepoint from the analysis. Probabilistic
                                   independent component analysis was applied using MELODIC (model-free fMRI analysis using
                                   probabilistic independent component analysis) and components resembling movement were removed.
                                       Time-series analysis was performed using a general linear model (GLM) by convolving the
                                   experimental design with either a standard adult hemodynamic response function (HRF) or a neonatal-
                                   specific HRF (Arichi et al., 2012). This approach was used to identify voxels in the brain that have
                                   a significantly increased level of BOLD activity (threshold at z = 2.3 with a corrected cluster
                                   significance level of p < 0.05). Group analyses were performed separately on adults and infants, and
                                   were performed independently for each stimulus intensity.
                                       A voxel-based conjunction analysis was not performed between adult and infant participants
                                   because of extreme differences between infant and adult brain anatomy, which would make such an
                                   analysis unreliable. As the insula is a key region of interest in nociceptive processing the distribution of
                                   activity within the insula was also reported.

                                   Identifying active regions
                                   Anatomical brain regions were classified using the Adult Harvard–Oxford cortical and subcortical
                                   atlases (Desikan et al., 2006) and a neonatal-specific atlas, which uses similar anatomical
                                   nomenclature as the Harvard–Oxford atlases (Shi et al., 2011). As the cerebellum was not identified
                                   in either the adult or neonatal atlas, and the brainstem not identified in the neonatal atlas, therefore
                                   masks (which were available as part of the standard templates in each population) were used to
                                   identify these regions. Comparisons between the infant and adult brain activity was considered on
                                   a gross anatomical scale. For example, the temporal gyrus, visual cortex, and brainstem were each
                                   considered as single structures. Active brain regions were defined as regions with more than one
                                   active voxel with significant positive BOLD activity (thresholded at z = 2.3 with a corrected cluster
                                   significance level of p < 0.05). A conversion from standard to functional space was performed to
                                   calculate the number of active voxels.

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                                      In adults, the FSL function Atlasquery was used to generate a list of active brain regions at each
                                   stimulus intensity, based on the masked clusters in the Harvard–Oxford cortical and subcortical atlases
                                   (Desikan et al., 2006). The use of Atlasquery ensured that all active voxels were identified in each brain
                                   region. The percentage of active voxels in each anatomical mask was then used to threshold regions,
                                   such that only regions with more than one active voxel were identified as active. The FSL function Cluster
                                   was used to identify the peak z statistic and MNI coordinates in each active region (see Table 1).
                                      In infants, the active brain regions were identified using MATLAB. Infant activity at each stimulus
                                   intensity level (thresholded at z = 2.3 with a corrected cluster significance level of p < 0.05) was used
                                   to mask the neonatal atlas (Shi et al., 2011). The masked image was imported into MATLAB so that
                                   each active region could be identified and the number of active voxels within each region calculated in
                                   the neonatal atlas space. A conversion from standard to functional space allowed quantification of the
                                   number of active voxels in the infant functional space and brain regions with more than one active
                                   voxel were identified as active. The FSL function Cluster was used to identify peak z statistics and
                                   coordinates in neonatal template space for each active region (Table 1).

                                   Percentage BOLD increase in active anatomical brain regions
                                   Once the active brain regions were identified at each stimulus intensity, an activity mask was created
                                   for each brain region based on the group analysis of all inputs across all stimulus intensities
                                   (z threshold = 2.3) for both the adults and infants. The activity mask was separated into anatomical
                                   regions of interest (based on brain regions which had been identified as active) and using Featquery
                                   the parameter estimate of the average percentage BOLD increase within each mask for each
                                   participant at each stimulus intensity was calculated.

                                   Statistics
                                   MRI data—intensity analysis
                                   Regression analysis was carried out using the software packages Graphpad Prism & R. Mean
                                   percentage signal change was plotted against stimulus intensity and regression analysis was used to
                                   test the null hypothesis that no intensity encoding was present within the masked activity within each
                                   anatomical brain region. A Bonferroni correction for multiple comparisons was used to determine the
                                   p threshold required in order to reject the null hypothesis. The corrected significance threshold was p
                                   = 0.00156. Parameter estimates for the gradient of the regression were used to rank brain regions
                                   that were active and exhibited significant intensity encoding.

                                   Adult psychophysics
                                   The mean pain score across each train of 10 stimuli at each stimulus intensity was calculated. The
                                   relationship between the mean pain scores and stimulus intensity was quantified using linear
                                   regression.

                                   Acknowledgements
                                   This work was funded by the Wellcome Trust. Sezgi Goksan is a MRC funded DPhil student. We would
                                   like to thank Eugene Duff, Jelena Bozek Mouthuy, Gabriela Schmidt Mellado, Sheula Barlow, Gabrielle
                                   Green, Falk Eippert, David Parker, and Caroline Young for their analytical, clinical and technical
                                   support. We would also like to thank the infants, adults, and parents who took part in this study.

                                   Additional information
                                   Funding
                                   Funder                           Grant reference                   Author
                                   Wellcome Trust                   Wellcome Trust Career             Rebeccah
                                                                    Development Fellowship,           Slater
                                                                    WT095802MA
                                   Medical Research Council         Graduate Student Fellowship       Sezgi Goksan
                                   (MRC)

                                   The funders had no role in study design, data collection and interpretation, or the
                                   decision to submit the work for publication.

Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                          11 of 13
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                                   Author contributions
                                   SG, RS, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or
                                   revising the article; CH, RP, Acquisition of data, Analysis and interpretation of data, Drafting or
                                   revising the article; FE, Conception and design, Acquisition of data; NC, FM, Analysis and
                                   interpretation of data, Drafting or revising the article; RR, EA, IT, Conception and design, Analysis
                                   and interpretation of data, Drafting or revising the article; JC, MS, Acquisition of data, Drafting or
                                   revising the article; SC, Conception and design, Acquisition of data, Analysis and interpretation of
                                   data; MJ, Acquisition of data, Analysis and interpretation of data
                                   Author ORCIDs
                                   Caroline Hartley,      http://orcid.org/0000-0002-7981-0836
                                   Ethics
                                   Human subjects: Informed written consent and consent to publish was provided by adult participants
                                   or by the infant’s parents. The study was approved by the Oxford and South Central Research Ethics
                                   Committees of the National Research Ethics Service and the University of Oxford Central University
                                   Research Ethics Committee (CUREC) (refs.: Investigating pain in the developing human brain; study
                                   number: 12/SC/0447; Human pain perception; study number: 11/SC/0249; CUREC study number:
                                   MSD/IDREC/C1/2011/143). The study conformed to the standards set by the Declaration of Helsinki
                                   and Good Clinical Practice guidelines.

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Goksan et al. eLife 2015;4:e06356. DOI: 10.7554/eLife.06356                                                                                      13 of 13
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